1. Field of The Invention
This invention relates to well cementing. This invention further relates to cementing wells which penetrate subterranean formations which produce gas. This invention still further relates to a composition for and a method of cementing a well with a slurry of hydraulic cement in water whereby the movement of gas into the slurry from a subterranean formation adjacent thereto is substantially reduced if not eliminated.
2. Related Art and Problem Solved
In the art of cementing a wellbore penetrating a subterranean earth formation, it is well known that a sheath of hardened cement is formed in the annular space between the walls of the wellbore which penetrates the formation and a well pipe, such as a casing, wherein the purpose of the sheath is to support the casing in the wellbore and to prevent the undesirable movement of formation fluids, i.e., oil, gas and water, within the annular space between subsurface formations and/or to the surface of the earth. It is known that the process of forming the sheath is referred to as primary cementing.
According to the well known process of primary cementing, a slurry of hydraulic cement in water is formed, the slurry is then pumped down the casing and then circulated up from the bottom thereof in the annulus to a desired location and then permitted to set in the annulus where the hydraulic cement reacts with the water in the slurry to form the sheath of hardened cement.
The slurry of cement, when first placed in the annulus, acts as a true liquid and will, therefore, transmit hydrostatic pressure. Thus, sufficient hydrostatic pressure is exerted, as a feature of the process of primary cementing, to balance the pressure of any gas in the formation to thereby prevent the movement of gas from the formation into and through the slurry in the annulus. Movement of gas from a formation into and through a cement slurry in an annulus is referred to in the art as gas migration.
Gas migration is a problem in primary cementing which can lead to movement of gas in the slurry from one formation to another or even to the surface of the earth. Such movement can cause loss of control of pressure and lead to a blowout. As mentioned previously, gas migration can be controlled if sufficient pressure can be transmitted through the slurry. However, loss of control can be experienced and gas migration can occur if the slurry does not possess the properties of a true fluid and is unable to transmit hydrostatic pressure.
Before a slurry of hydraulic cement sets into a hardened mass having compressive strength, events take place which cause the slurry to lose the ability to transmit hydrostatic pressure. One of the events is the loss of fluid from the slurry to the formation. Another event is the development of static gel strength in the slurry.
It seems clear that the loss of water from a slurry of cement will diminish the ability of the slurry to transmit hydrostatic pressure. The ability to control water loss is more difficult as the temperature increases, especially at temperatures greater than about 200.degree. F. It is thus an object of this invention to provide a composition for and a method of reducing fluid loss from a slurry of hydraulic cement at temperatures greater than about 200.degree. F.
When a slurry of hydraulic cement becomes static it begins to develop a property known in the art as static gel strength, or simply gel strength. (In this regard, note Sabins, et al., "The Relationship of Thickening Time, Gel Strength, and Compressive Strength of Oilwell Cements," SPE Production Engineering, March 1986, pages 143-152.)
Gel strength is not compressive strength. Thus, as a slurry of hydraulic cement sets into a hardened mass having compressive strength, it is believed that the hardening process experiences phases which are relevant to the phenomenon of gas migration. In the first phase of the process, it is believed that the slurry contains sufficient liquid to enable the slurry to possess the characteristics of a true fluid. Accordingly, during the first phase, the slurry can transmit hydrostatic pressure and gas migration can be prevented by applying sufficient hydrostatic pressure which is transmitted against a gas-containing formation to thereby prevent the movement of gas from the formation into the slurry.
During the first phase of the process, some of the liquid in the slurry is lost--this is referred to as fluid loss--and the slurry begins to stiffen due to the formation of a gel structure. During this period of fluid loss and gel formation it is believed that the setting cement retains the ability to transmit hydrostatic pressure. Accordingly, gas migration can be prevented so long as the slurry exhibits the properties of a true fluid and so long as the stiffness of the gel structure--referred to as gel strength--is less than or equal to a certain value, which, for purposes of this invention, is referred to as the first critical value. The first critical value is believed to be about 100 lb.sub.f /100 sq. ft.
In the second phase of the hardening process, the gel strength of the slurry exceeds the first critical value and continues to increase; fluid loss may continue, although at a rate much lower than that experienced in the first phase. During this period, it is believed that the setting cement loses the ability to transmit full hydrostatic pressure. Accordingly, gas migration may not be prevented during the second phase because the gel strength of the slurry may be too high to permit full transmission of hydrostatic pressure, but too low to resist pressure exerted by gas in the formation against the slurry. This condition exists until the gel strength increases to a value, which for purposes of this invention, is referred to as the second critical value, which is high enough to resist pressure exerted by gas in the formation against the slurry. The second critical value is believed to be about 500 lb.sub.f /100 sq. ft.
In the third phase of the hardening process, gas migration is prevented because gel strength is equal to or greater than the second critical value. The cement continues to harden until it attains a compressive strength deemed sufficient to enable further operations in the wellbore.
It is noted that Sabins, et al., mentioned above, provide a discussion and a description of a method and apparatus to experimentally determine gel strength value.
In view of the above, in order to minimize gas migration, it is desirable that maximum fluid loss, if any, should occur prior to the beginning of the second phase of the cement hardening process; that the first phase should continue over an extended period of time; and that the second phase should be completed in a short period of time.
The time required for a slurry of hydraulic cement to attain the first critical value from the time the slurry becomes static is defined herein as "Zero Gel Time," and the time required for a slurry to attain the second critical value from the time it attains the first critical value is defined herein as "Transition Time."
It is thus another object of this invention to provide a composition for and a method of extending Zero Gel Time of a slurry for a time sufficient to enable the rate of fluid loss from the slurry to decline to a substantially constant value and to accelerate Transition Time.
It is a further object of this invention to provide a method of cementing a borehole which penetrates a gas-containing subterranean formation whereby gas migration at temperatures up to 400.degree. F. and particularly above 200.degree. F. is reduced if not eliminated.